Method and apparatus for producing a laminar printing form



Dec. 2, 1969 F. P. VALENTIN E L METHOD AND APPARATUS FOR PRODUCING A LAMINAR PRINTING FORM Filed May 6, 1966 R R E o F m m L .L R C S O m 3 1 R T W F l R U EE 9 SM ..r m NE R mm EH R Y TD S u 1 8 L 34 o 1 .l

5 1 W m g H O L ,4 w 1 BMW R 2 m mm 0 PF 1. W TL UP OM A log -. OUTPUT I NVEN TOR S OSCILLATOR Franeois Valeni Yves Basso Charles P. 'Rong|er ATTORNEYS United States Patent 3,482,039 METHOD AND APPARATUS FOR PRODUCING A LAMINAR PRINTING FORM Francois P. Valentin, Bordeaux, Yves Basso, Camblanes, and Charles P. Rongier, Bordeaux, France, assignors to Zeuthen & Aagaard A/ S, Glostrup, Denmark Filed May 6, 1966, Ser. No. 548,306 Claims priority, application France, May 13, 1965,

Int. C]. 1304,] 5/76 US. Cl. 1786.6 12 Claims ABSTRACT ,OF THE DISCLOSURE A method and apparatus for perforating stencils electrically to achieve half-tone prints in which the threshold value is changed systematically while analyzing the original to produce a half-tone stencil.

This invention relates to a method for producing a laminar printing form, that is a printing form comprising one or more laminae and is concerned with the production of such forms in which a lamina is perforated. The lamina blank can be perforated electrically in which case it is called an electro-stencil, but for off-set forms only one lamina of say two may be perforated.

An apparatus for the electrical production of stencils from black-white originals is known, in which an original to be reproduced is scanned by means of a photo-electric cell, for example a photo-multiplier, the output voltage of which across a load resistor is proportional to the quantity of light reflected by the original, whilst an electrical circuit arrangement with adjustable threshold controls the voltage on a stylus in such a manner that the stencil is perforated when the numerical value of the output voltage of the photocell is lower than that of the threshold.

These apparatuses are not well suited to produce grey-tones, especially not on the so-called homogeneous electro-stencils which are the cheapest and most used stencils, and it was, therefore, suggested to alter the threshold value during the scanning of the original.

It is an object of the invention to improve the method of producing a laminar form as defined above.

More specifically it is an object of the invention to improve the homogeneity of the reproduction of greytone pictures combined with an increase in the speed of production.

The invention is characterised in adding to the output voltage of the photo-electric cell a saw-tooth voltage of appropriate frequency and amplitude.

In most apparatuses for the electrical production of stencils, the perforation or cutting voltage on the stylus is not a DC. voltage but an A.C. voltage so that the stencil is cut away spotwise or discontinuously, which improves the mechanical resistivity of the stencil but reduces the range of the grey values reproduced in relation to that present in the original and, consequently, this involves compression of the grey-tone scale. Furthermore, experience shows that the reproduction is more satisfactory when this compression is not even but greater in the medium grey-tone range where the eye does not require so high contrast than at the very dark or very light areas where the eye requires good contrast.

A further feature of the invention consists in that the characteristic representing the relation between the amplitude of the saw-tooth voltage added to the output voltage of the photocell and time, has a comparatively slight steepness near the maximum and minimum values and a greater steepness in the range of medium values.

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As the variation in the output voltage of the photocell device for one and the same difference in grey-tone values is much smaller in the dark range than in the bright range, the characteristic of the photocell device must be corrected so that the sensitivity is equalized within the whole range of grey values. According to the invention, this is obtained by including in the circuit of the photocell device a non-linear resistor, the resistance of which varies substantially inversely proportional with the intensity of the current through the resistor.

In a suitable embodiment of the apparatus according to the invention, the said non-linear resistor comprises at least one diode working in the forward direction. The non-linear resistor comprises preferably several silicon diodes in series.

However, it is only possible for the laminar form to a limited extent to reproduce in the perforation pattern the large variation range of the output signal. In order to obtain a reasonable fine graduation of grey values in the dark range, which is the most critical range to the eye, a compression of the output signals within the central range and the bright range of the grey-tone scale and an expansion of the output signals within the dark range of the grey-tone scale is necessary. According to the inventionthis is obtained by substantially diminishing the steepness of the lower part of the saw-tooth curve thus enhancing the details in the reproduction of the darker range. A suitable diminishing of the steepness of the curve representing the saw-tooth voltage in the neighbourhood of the minimum amplitude can easily be obtained by the choice of a suitable time constant.

The period of the saw-tooth signal should be short with a view to maintaining a good definition in the reproduction but should on the other hand comprise a sufficiently large number of periods of the generally sinusoidal cutting signal defining the cutout points in order to attain an appropriate graduation of the grey value range reproduced. It is preferred that the period of the sawtooth signal covers from three to five periods of said cutting signal.

It is of advantage if the ratio between the number of cycles of the saw-tooth voltage and that of the cutting signal is simple, and that the saw-tooth voltage is synchronized with the cutting signal to avoid beating which would produce a moir pattern in the reproduction whereas no systematic streaks will occur in the reproduction when there is no simple ratio between the frequency of said saw-tooth voltage and the line frequency of the scanning mechanism.

The invention will be particularly explained below with reference to the accompanying schematical drawing, in which- FIG. 1 shows a circuit diagram of an apparatus known per se for the electrical reproduction of originals on stencils,

FIG. 2 is a circuit arrangement according to the in vention for an apparatus of the type shown in FIG. 1, and

FIGS. 3 and 4 are diagrams to explain the mode of operation of the circuit arrangement.

The apparatus known per se whose electric circuit is shown in FIG. 1, comprises a mechanism for synchronised feeding of the original to be reproduced and the electrostencil to be perforated, a photo electric device such as a photocell or photo-multiplier associated with a lens system 'focussed on the original, and an electrode for cutting the stencil by sparks. The original and the stencil are, for example, disposed side by side on a rotary drum, and a slide which carries the lens system and the photocell at one end and the electrode at the other end moves parallel to the drum axis in such a way that the original is scanned and reproduced line by line. Vice versa the drum may be advanced in the axial direction, too, while the lens system and the photocell are stationary.

The circuits shown in FIG. 1 serve to apply a potential to an electrode 1 for perforating a stencil 2 when the small area or spot 3 of the original which at a given time is scanned by means of the lens system 4, is darker than a grey value chosen as threshold value. These circuits comprise a circuit arrangement 5 for rectifying and stabilizing the main voltage for feeding the apparatus with the required D.C. voltages, an oscillator 6 supplying the cutting or spark voltage of a suitable frequency, a photomultiplier 7 associated with the lens system 4, an amplifier with threshold or a trigger device 8 which receives both the generally sinusoidal voltage from the oscillator 6 across a potentiometer 9 and the signal occurring across the load resistor 10 of the photo-multiplier to which signal a voltage adjustable by means of a potentiometer 11 is added for adjusting the threshold, and an output amplifier 12 feeding the electrode 1. The working voltage of the photo-multiplier is provided by means of a rectifier 13 fed from the oscillator 6 which also via a variable resistor 15 feeds the lamps 14 for illuminating the spot 3.

The circuit arrangement according to the invention shown in FIG. 2 replaces the load resistor 10, shown in FIG. 1, for the photo-multiplier 7 and the potentiometer 11 for adjustment of the threshold, but in the interest of clearness the oscillator 6, the photo-multiplier 7, the trigger device 8, the potentiometer 9, the output amplifier 12 and the rectifier 13 are also shown.

The photo-multiplier 7, of which only a photocathode 70, which receives the light beams coming from the lens system 4, dynodes 71, 72 78 and an anode 79 are shown, operates with the negative voltage applied to the cathode through the rectifier 13. In the apparatus shown in FIG. 1 where the load resistor of the photo-multiplier is linear, the photo-multiplier feeds to the input of the trigger device 8 a negative voltage the numerical value of which is approximately proportional to the quantity of light reflected and collected by the lens system and passed on to the photocathode. The trigger device 8 comprises an input tube 81, for example a pentode operating as an amplifier which changes from the blocked condition to the saturated condition by a sufficient change in the voltage of the output signal of the photo-multiplier 7 on the control grid of the tube 81, and a double triode 82 with a common cathode resistor for the two triodes 82a and 82b. The control grid of the triode 82a is connected to the andoe of the pentode 81, and the control grid of the triode 82b is connected to the output of the oscillator 6 via the potentiometer 9. When the intensity of the light received by the photo-multiplier is sufficient to block the pentode 81, the high anode voltage on the pentode causes saturation of the triode 82a, and the resulting increase of the common cathode voltage on the triodes 82a and 82b blocks the latter triode so that the signal applied to the control grid of the triode 82b is not transmitted to the output amplifier 12 via the anode of the triode. When the intensity of the light received by the photo-multiplier is not suflicient to block the pentode 81, the triode 82a will be blocked or only slightly conductive, and the triode 82!) transmits the amplified signals to the output amplifier 12 which increases the voltage of the electrode 1 to the desired value for perforating the stencil 2.

According to the invention a saw-tooth voltage is introduced into the anode load circuit of the photo-multiplier 7 and thus added to the signal supplied by the photomultiplier. The combined signal varies periodically in accordance with the saw-tooth voltage and in relation to the threshold voltage necessary to trigger the trigger device 8 whereby such pattern of perforation in the stencil is produced that grey-tones occur in the reproductions made by the stencil. The saw-tooth voltage is suitably deformed to attain greater contrast in the lighter and the darker areas than in the medium value areas.

The saw-tooth signal is generated by discharging a capacitor 21 through a unijunction transistor 22 which is fed via a voltage divider consisting of two resistors 23 and 24 each of which is connected to an elect-rode in the transistor, and these electrodes are interconnected via a Zener diode 25 which fixes the voltage between said electrodes and, consequently, reduces the thermal drift of the transistor. The saw-tooth signals are synchronized with the sinusoidal signal from the oscillator 6, by means of a capacitor 26 connecting the tap of the potentiometer 9 to the base of the transistor 22. The saw-tooth output signals from the transistor are via a capacitor 27 and a diode 28 transmitted to a capacitor 29, the discharge circuit of which comprises a resistor in series with a parallel combination of a resistor 31 and a Zener diode 32. The time constant for the discharge of the capacitor 29 through the series-connected resistors 30 and 31 serves to reduce the steepness of the lower part of the saw-tooth curve, and the series combination of the Zener diode 32 and resistor 30 results in a progressive clipping of the curve. The signal produced across the resistor 31 has, as shown in FIG. 3, the shape of an S. The lower level of the signal is made independent of the amplitude of the signal by means of a diode 33 having the cathode connected to the anode of the diode 28 at the input of the circuit deforming the saw-tooth signal to an S-shaped signal, and the anode connected to the common point of the circuit. The voltage at this common point may be adjusted by means of the potentiometer 34 shunted by a capacitor 35 and grounded via the parallel combination of a capacitor 37 and a temperature-responsive resistor 36, the purpose of which will be explained further below.

The deformed saw-tooth signal derived from the resistor 31 is supplied to the anode of the photo-multiplier 7 via a potentiometer tap 310 by means of which the amplitude of the signal can be adjusted, and a row 20 of series-connected diodes operating in the forward direction. These diodes are preferably silicon diodes whose characteristic showing the relation between the resistance and the voltage applied has a break at about 0.6 volt, and a suitable number of diodes, for example 15, are used for obtaining a sufiiciently high voltage at the output of the photo-multiplier.

The voltage-dependent variation of the resistance of the diodes 20 compensates for the lack of linearity in the characteristic of the photo-multiplier showing the relation between the output voltage and the grey-tone value of the original which varies logarithmically.

Thus, the signal at the input of the trigger device 8 is the sum of the output voltage V of the photo-multiplier and the deformed saw-tooth signal transmitted to the input of the diode row 20 and further deformed as a consequense of the lack of linearity of the equivalent resistance of the diodes. FIG. 4 shows the relation between the intensity of the light reflected from the area scanned on the original, and the sum signal as a function of time 2. Reference a corresponds to the light areas, b corresponds to the dark areas, and 0 corresponds to medium values.

The temperature-responsive resistor 36 compensates posed directly near the diodes 20 and is, consequently, exposed to the same ambient temperature, and the capacitor 37 parallel-connected to the resistor 36 is an electrochemical capacitor of high capacity which serves to prevent a disturbing capacitive coupling from occu ring between the temperature-responsive resistor 36, through which a variable current flows, and the diodes 20 as a consequence of said components lying close to each other.

From the foregoing description it will appear that the reproduction of the grey-tones in the above mentioned method and in the apparatus described does not depend on the density of the scanning lines, and experience shows that in proceeding in conformity with the invention very satisfactory reproduction will be attained with a density of scanning lines, that is much smaller, for example five times smaller, than in the heretofore known methods and apparatuses.

As the time required for producing the stencil is proportional to the number of successively disposed lines which are to be scanned for analyzing the original to be reproduced, the reduction of the definition thus attained will make possible a corresponding reduction of the time of production and, consequently, increase the production.

The reduction of the density of lines required for a very good reproduction further affords the advantage that the number of perforations in the stencil and, consequently, the quantity of ink consumed for the printing, will be proportionately reduced. As a consequence of this saving of ink, the quantity of ink applied to the paper during the printing process will for the darkest areas of the original substantially remain within the limits permitted by the absorption qualities of the paper and the evaporation of the ink, which means that smearing of the reverse of newly printed copies which at the outlet of the duplicating machine are stacked on top of each other, is avoided or considerably reduced, which again means that the interposition of protective sheets can be avoided.

We claim:

1. A method of producing a laminar printing form comprising the steps of scanning an original along a succession of scanning lines to generate electric signals representative of the grey-values along the lines, synchronously scanning a laminar form blank with a perforating device, applying the electric signals to the perforating device to cause it to perforate the blank when the grey-value in the original exceeds a given amount and, during scanning, varying the grey-value at which perforation takes place by adding to said electric signal a continuously varying electric signal comprising a saw-tooth voltage.

2. A method as claimed in claim 1, wherein said continuously varying electric signal is a saw-tooth voltage.

3. A method as claimed in claim 1, wherein said continuously varying electric signal is a modified saw-tooth voltage of Which the curve showing the relation between voltage and time is less steep in the neighborhood of the maximum and minimum values of the saw-tooth pulses and more steep in the intermediate range.

4. A method as claimed in claim 1, wherein the generated electric signals supplied to the perforating device include an alternating voltage and wherein the frequency of the voltage exceeds the repetition frequency of the saw-tooth voltage preferably in a ratio between 1:3 and 1:5.

5. A method as claimed in claim 1, wherein the generated electric signals supplied to the perforating device include an alternating voltage, and the ratio between the frequency of said voltage and the repetition rate of the saw-tooth pulses is a simple number.

6. A method as claimed in claim 1, wherein said generated electric signals are modified into a voltage signal varying as a substantially linear function of incident light varying substantially logarithmically.

7. An apparatus for producing a laminar printing form comprising means for scanning an original along a succession of scanning lines to generate electric signals representative of the grey-values along the lines, means for synchronously scanning a laminar form blank with a perforating device, means for applying the electric signals from said scanning means to the perforating device causing the latter to perforate the blank when the grey-value in the original exceeds a given amount and means for continuously varying, during scanning, the grey-value at which perforation takes place, said last mentioned means comprising a saw-tooth generator.

8. An apparatus as claimed in claim 7, further comprising means for modifying the shape of the saw-tooth curve into a form having less steepness in the neighborhood of maximum and minimum amplitude and greater steepness in the intermediate range.

9. An apparatus as claimed in claim 7, further comprising means for synchronizing the frequency of the electric signals generated by said scanning means and the frequency of the saw-tooth voltage generated by said saw-tooth generator.

10. An apparatus as claimed in claim 7, wherein said scanning means comprises a photo-multiplier and said means for continuously varying comprises means for modifying the output current of said photo-multiplier which is normally a linear function of the incident light so as to set up an output voltage varying in a substantially linear way as a function of logarithmically varying incident light.

11. An apparatus as claimed in claim 10 wherein the means for continuously varying comprises at least one diode.

12. An apparatus as claimed in claim 10 wherein the means for continuously varying comprises a number of diodes connected in series.

References Cited UNITED STATES PATENTS 2,777,058 1/1957 Boyajean 25027 2,881,246 4/1959 Fairchild 1786.6 3,043,906 7/1962 Hassing 178-6.6 3,393,269 7/1968 Zeuthen 178--6.6

ROBERT L. GRIFFIN, Primary Examiner R. K. ECKERT, JR., Assistant Examiner 

